util: netdevbridge: fall back to ioctl from sysfs

util: netdevbridge: fall back to ioctl from sysfs

There are certain cases e.g. containers where the sysfs path might
exists, but might fail. Unfortunately the exact restrictions are only
known to libvirt when trying to write to it so we need to try it.

But in case it fails there is no need to fully abort, in those cases try
to fall back to the older ioctl interface which can still work.

That makes setting up a bridge in unprivileged LXD containers work.

Fixes: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1802906
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Laine Stump <laine@laine.org>
Reported-by: Brian Candler <b.candler@pobox.com>